1. Field of the Invention
This invention relates to the field of integrated optical devices, and in particular to a method of reducing stress-induced mechanical problems in optical components, especially optical components that are deep-etched, such as multiplexers and demultiplexers employing echelon gratings.
2. Description of the Related Art
The manufacture of integrated optical devices such as optical Multiplexers (Mux) and Demultiplexers (Dmux) requires the fabrication optical quality elements, such as waveguides and gratings highly transparent in the 1.30 and 1.55 μm optical bands. These silica-based optical elements are basically composed of three films: buffer, core and cladding. For reasons of simplicity, the buffer and cladding are typically of the same composition and of same refractive index. In order to confine the 1.55 μm (and/or 1.30 μm) wavelength laser beam, the core must have a higher refractive index than the buffer (cladding). This required refractive index difference is called the ‘delta-n’ and is one of the most important characteristics of these silica-based optical elements. It is very difficult to fabricate such transparent silica-based optical elements in the 1.55 μm wavelength (and/or 1.30 wavelength) optical region while maintaining the suitable ‘delta-n’ and while preventing stress-induced mechanical and problems.
Our co-pending U.S. patent application Ser. No. 09/867,772 entitled “Method of Depositing Optical Quality Films”, describes an improved Plasma Enhanced Chemical Vapour Deposition technique of these silica-based elements which allows the achievement of the required ‘delta-n’ while eliminating the undesirable residual Si:N—H oscillators (observed as a FTIR peak centered at 3380 cm−1 whose 2nd harmonics could cause an optical absorption between 1.445 and 1.515 μm), SiN—H oscillators (centered at 3420 cm−1 whose 2nd harmonics could cause an optical absorption between 1.445 and 1.479 μm) and SiO—H oscillators (centered at 3510 cm−1 and whose 2nd harmonics could cause an optical absorption between 1.408 and 1.441 μm) after a high temperature thermal treatment in a nitrogen ambient at 800° C.
Another co-pending patent application, Ser. No. 09/956,916, filed on Sep. 21, 2001, entitled “Method of Depositing an Optical Quality Silica Film by PECVD”, shows that to such a high temperature thermal treatment are associated some residual stress-induced mechanical problems of deep-etched optical elements (mechanical movement of the side-walls), some residual stress-induced mechanical problems at the buffer/core interface or at the core/cladding interface (micro-structural defects, micro-voiding and separation) and some residual stress-induced optical problems (polarisation dependant power loss) which can be eliminated by an improved process allowing the simultaneous optimization of the optical and of the mechanical properties of buffer (cladding) and core in a seven-dimensional space, namely a first independent variable, the SiH4 flow; a second independent variable, the N2O flow; a third independent variable, the N2 flow; a fourth independent variable, the PH3 flow; a fifth independent variable, the total deposition pressure; a sixth independent variable, the optimised post-deposition thermal treatment; and the observed silica-based optical elements characteristics.
Recently published literature reveals various PECVD (Plasma Enhanced Chemical Vapor Deposition) approaches to obtain these high performance optically transparent silica-based optical elements: Valette S., New integrated optical multiplexer-demultiplexer realized on silicon substrate, ECIO '87, 145, 1987; Grand G., Low-loss PECVD silica channel waveguides for optical communications, Electron. Lett., 26 (25), 2135, 1990; Bruno F., Plasma-enhanced chemical vapor deposition of low-loss SiON optical waveguides at 1.5-μm wavelength, Applied Optics, 30 (31), 4560, 1991; Kapser K., Rapid deposition of high-quality silicon-oxinitride waveguides, IEEE Trans. Photonics Tech. Lett., 5 (12), 1991; Lai Q., Simple technologies for fabrication of low-loss silica waveguides, Elec. Lett., 28 (11), 1000, 1992; Lai Q., Formation of optical slab waveguides using thermal oxidation of SiOx, Elec. Lett., 29 (8), 714, 1993; Liu K., Hybrid optoelectronic digitally tunable receiver, SPIE, Vol 2402, 104, 1995; Tu Y., Single-mode SiON/SiO2/Si optical waveguides prepared by plasma-enhanced Chemical vapor deposition, Fiber and integrated optics, 14, 133, 1995; Hoffmann M., Low temperature, nitrogen doped waveguides on silicon with small core dimensions fabricated by PECVD/RIE, ECIO'95, 299, 1995; Bazylenko M., Pure and fluorine-doped silica films deposited in a hollow cathode reactor for integrated optic applications, J. Vac. Sci. Technol. A 14 (2), 336, 1996; Poenar D., Optical properties of thin film silicon-compatible materials, Appl. Opt. 36 (21), 5112, 1997; Hoffmann M., Low-loss fiber-matched low-temperature PECVD waveguides with small-core dimensions for optical communication systems, IEEE Photonics Tech. Lett., 9 (9), 1238, 1997; Pereyra I., High quality low temperature DPECVD silicon dioxide, J. Non-Crystalline Solids, 212, 225, 1997; Kenyon T., A luminescence study of silicon-rich silica and rare-earth doped silicon-rich silica, Fourth Int. Symp. Quantum Confinement Electrochemical Society, 97-11, 304, 1997; Alayo M., Thick SiOxNy and SiO2 films obtained by PECVD technique at low temperatures, Thin Solid Films, 332, 40, 1998; Bulla D., Deposition of thick TEOS PECVD silicon oxide layers for integrated optical waveguide applications, Thin Solid Films, 334, 60, 1998; Valette S., State of the art of integrated optics technology at LETI for achieving passive optical components, J. of Modern Optics, 35 (6), 993, 1988; Ojha S., Simple method of fabricating polarization-insensitive and very low crosstalk AWG grating devices, Electron. Lett., 34 (1), 78, 1998; Johnson C., Thermal annealing of waveguides formed by ion implantation of silica-on-Si, Nuclear Instruments and Methods in Physics Research, B141, 670, 1998; Ridder R., Silicon oxynitride planar waveguiding structures for application in optical communication, IEEE J. of Sel. Top. In Quantum Electron., 4 (6), 930, 1998; Germann R., Silicon-oxynitride layers for optical waveguide applications, 195th meeting of the Electrochemical Society, 99-1, May 1999, Abstract 137, 1999; Worhoff K., Plasma enhanced cyhemical vapor deposition silicon oxynitride optimized for application in integrated optics, Sensors and Actuators, 74, 9, 1999; Offrein B., Wavelength tunable optical add-after-drop filter with flat passband for WDM networks, IEEE Photonics Tech. Lett., 11 (2), 239, 1999.
A comparison of these various PECVD techniques is summarised in FIG. 1 which describes the approaches and methods used to modify the ‘delta-n’ between buffer (cladding) and core with a post-deposition thermal treatment.
The various techniques can be grouped in main categories: PECVD using unknown chemicals, unknown chemical reactions and unknown boron (B) and/or phosphorus (P) chemicals and unknown chemical reactions to adjust the ‘delta-n’ (When specified, the post-deposition thermal treatments range from 400 to 1000° C.); PECVD using TEOS and unknown means of adjusting the ‘delta-n’ (The post-deposition thermal treatments are not specified); PECVD using oxidation of SiH4 with O2 coupled with silicon ion implantation or adjustment of silicon oxide stoichiometry as means of adjusting the ‘delta-n’ (The post-deposition thermal treatments range from 400 to 1000° C.) PECVD using oxidation of SiH4 with O2 coupled with the incorporation of CF4 (SiH4/O2/CF4 flow ratio) an means of adjusting the ‘delta-n’ (Wen specified, the post-deposition thermal treatments range from 100 to 1000° C.) PECVD using oxidation of SiH4 with N2O coupled with variations of N2O concentration (SiH4/N2O flow ratio) as means of adjusting the silicon oxide stoechiometry and the ‘delta-n’ (The post-deposition thermal treatments range from 400 to 1100° C.); PECVD using oxidation of SiH4 with N2O coupled with variations of N2O concentration and with the incorporation of Ar (SiH4/N2O/Ar flow ratio) as means of adjusting the silicon oxide stoechiometry and the ‘delta-n’ (The post-deposition thermal treatments is 1000° C.); PECVD using oxidation of SiH4 with N2O coupled with the incorporation of NH3 (SiH4/N2O/NH3 flow ratio) as to form silicon oxynitrides with various ‘delta-n’ (When specified, the post-deposition thermal treatments range from 700 to 1100° C.); PECVD using oxidation of SiH4 with N2O coupled with the incorporation of NH3 and Ar (SiH4/N2O/NH3/Ar flow ratio) as to form silicon oxynitrides with various ‘delta-n’ (The post-deposition thermal treatments are not specified); PECVD using oxidation of SiH4 with N2O coupled with the incorporation of NH3 and N2 chemicals variation (SiH4/N2O/NH3/N2 flow ratio) as to form silicon oxynitrides with various ‘delta-n’ (The post-deposition thermal treatments range from 850 to 1150° C.); and PECVD using oxidation of SiH4 with N2O and O2 coupled with the incorporation of CF4, N2 and He (SiH4/(N2O/N2)/O2/CF4 flow ratio) as to form complex mixtures of carbon and fluorine containing silicon oxide as means of adjusting the ‘delta-n’ (The post-deposition thermal treatments is 425° C.).
Our co-pending patent application Ser. No. 09/833,711 entitled “Optical Quality Silica Films” describes an improved Plasma Enhanced Chemical Vapour Deposition technique of low optical absorption buffer (cladding) which shows that the independent control of the SiH4, N2O and N2 gases as well as of the total deposition pressure via an automatic control of the pumping speed of the vacuum pump in a five-dimensional space: a first independent variable, the SiH4 flow; a second independent variable, the N2O flow; a third independent variable, the N2 flow; a fourth independent variable; the total deposition pressure (controlled by an automatic adjustment of the pumping speed); and the observed film characteristics is key to eliminating the undesirable residual Si:N—H oscillators (observed as a FTIR peak centered at 3380 cm−1 whose 2nd harmonics could cause an optical absorption between 1.445 and 1.515 μm), SiN—H oscillators (centered at 3420 cm−1 whose 2nd harmonics could cause an optical absorption between 1.445 and 1.479 μm) and SiO—H oscillators (centered at 3510 −1 and whose 2nd harmonics could cause an optical absorption between 1.408 and 1.441 μm) after thermal treatments at a low post-deposition temperature of 800° C. to provide improved silica films with reduced optical absorption in the 1.55 μm wavelength (and/or 1.30 wavelength) optical region.
Our co-pending patent application Ser. No. 09/867,662 entitled “Method of Depositing Optical Films” describes a new improved Plasma Enhanced Chemical Vapour Deposition technique of low optical absorption core which shows that the independent control of the SiH4, N2O, N2 and PH3 gases as well as of the total deposition pressure via an automatic control of the pumping speed of the vacuum pump in a six-dimensional space: a first independent variable, the SiH4 flow; a second independent variable, the N2O flow; a third independent variable, the N2 flow; a fourth independent variable, the PH3 flow; a fifth independent variable; the total deposition pressure (controlled by an automatic adjustment of the pumping speed); and the observed waveguides characteristics, is key to achieving the required ‘delta-n’ while still eliminating the undesirable residual Si:N—H oscillators (observed as a FTIR peak centered at 3380 cm−1 whose 2nd harmonics could cause an optical absorption between 1.445 and 1.515 μm), SiN—H oscillators (centered at 3420 cm−1 whose 2nd harmonics could cause an optical absorption between 1.445 and 1.479 μm) and SiO—H oscillators (centered at 3510 cm−1 and whose 2nd harmonics could cause an optical absorption between 1.408 and 1.441 μm) after thermal treatments at a low post-deposition temperature of 800° C. as to provide improved silica waveguides with reduced optical absorption in the 1.55 μm wavelength (and/or 1.30 wavelength) optical region.
Our co-pending application Ser. No. 09/956,916 filed on September 21, entitled “Method of Depositing an Optical Quality Silica Film by PECVD” describes a technique which shows that the simultaneous optimization of the optical and of the mechanical properties of buffer (cladding) and core in a seven-dimensional space: a first independent variable, the SiH4 flow; a second independent variable, the N2O flow; a third independent variable, the N2 flow; a fourth independent variable, the PH3 flow; a fifth independent variable, the total deposition pressure; a sixth independent variable, the optimised post-deposition thermal treatment; and the observed silica-based optical elements characteristics is key to achieving the required ‘delta-n’ while eliminating the undesirable residual Si:N—H oscillators (observed as a FTIR peak centered at 3380 −1 whose 2nd harmonics could cause an optical absorption between 1.445 and 1.515 μm), SiN—H oscillators (centered at 3420 cm−1 whose 2nd harmonics could cause an optical absorption between 1.445 and 1.479 μm) and SiO—H oscillators (centered at 3510 −1 and whose 2nd harmonics could cause an optical absorption between 1.408 and 1.441 μm) after an optimised thermal treatment in a nitrogen which can provide improved silica-based optical elements with reduced optical absorption in the 1.55 μm wavelength (and/or 1.30 wavelength) optical region without the residual stress-induced mechanical problems of deep-etched optical elements (mechanical movement of side-walls), without the residual stress-induced mechanical problems at the buffer/core or core/cladding interfaces (micro-structural defects, micro-voiding and separation) and without the residual stress-induced optical problems (polarisation dependant power loss).
Our co-pending pending patent application, Ser. No. 09/799,491 entitled Method Of Making A Functional Device With Deposited Layers Subject To High Temperature Anneal” describes a new improved technique involving the deposition of thick PECVD silica films on the back face of the silicon wafer in order to prevent the wafer warp problem following high temperature anneals and to achieve a stable manufacturing of high performance high temperature annealed PECVD optical silica films with lower polarisation dependence.
An object of the present invention is to an optimised process which allow the elimination of these residual stress-induced mechanical problems of deep-etched optical elements (mechanical movement of the side-walls), of these residual stress-induced mechanical problems at the buffer/core interface or at the core/cladding interface (micro-structural defects, micro-voiding and separation) and of these residual stress-induced optical problems (polarisation dependant power loss).